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of the ground waters and the formation of the NW-
SE branch of the karst galleries of Temnata Dupka
Cave (Table
2.4
). This direction is even contem-
porary active and this fact suggests a coincidence
of today stress axes orientations with these at the
end of the Jurassic.
The inferred age of initiation of the karst for-
mation process is 4.5 MA, i.e., from the beginning
of the Pliocene (Angelova et al.
1999
), and it was
probably facilitated by the fractures created or
reactivated during the next stage of deformations.
2. The second stage of deformation created a new
system of shear joints (diagram I b on Table
2.4
),
but some of the older joint surfaces were also
translators of the block movements. The striations
on the slickensides confirm the existence of this
tectonic stress field (diagram II on Table
2.4
). This
stress field favored the opening of the northeast-
wards striking shear joints of the older deformation
stage (see diagram I a on Table
2.4
) and their
transformation in fissures. The regional studies
(Chanov
1988
) have given reasons for the impact
of the Pyrenean Tectonic Phase on this process, as
it was discussed above for the specific features of
Vratsa Karst Region. It can be accepted that the
same regional tectonic stress conditions had
remained from the Oligocene to the Pliocene
Epoch, when the process of karst formation star-
ted. The northeast-southwest directed galleries of
the Temnata Dupka Cave reflect exactly this situ-
ation (Table
2.4
).
Fig. 2.36
Changes of the discharge rate of Iskrets Karst
Springs after Vrancea Earthquake of 04.03.1977, M = 7.2
(Paskalev et al.
1992
—Reproduced by permission of Bulletin of
Bulgarian Geological Society)
the discharge rate raised abruptly to 13.5 m
3
/s, and
began to decrease gradually (Fig.
2.36
). Similar event
happened during the local Svoge Earthquake of 1979,
as well as on the April 11, 1982, when any significant
earthquake was not recorded at the localities, nor in
the whole Balkan Region.
The question for the seismic impact is of impor-
tance for the local authorities because of the signifi-
cance of the springs as a major source of fresh water
for the municipality of the town of Svoge. The nor-
mally used quantities are of the rate of 150 l/s. The
impact from the blasts in the quarries has not the
potential to disturb the normal discharge of the springs
(Shanov et al.
2001
; Shanov and Benderev
2005
). The
only factor for such disturbances can be the local
tectonics and the dynamics of the processes inside the
karst system.
Iskrets Karst Springs have a very variable dis-
charge—from 280 to more than 50,000 l/s (Benderev
1989
). They drained more than 80 % of the territory
of Ponor Mountain (a part of Western Balkan
Mountain). The springs are related to the karst system
of the upper part of the Triassic Aquifer of Iskar
Carbonate Group. The atmospheric precipitations on
the feeding area of the springs (about 140 km
2
) pro-
vide the principal quantities of water—62 % of the
average annual discharge of the springs. Geological,
tectonic and geomorphological conditions predestine
the existence of water saturated zone north of the
springs. The underground lake surface inside the
neighboring Dushnika Cave has a level at 2 m above
the
Iskrets-Gubesh Karst Region (I-G)
Iskrets-Gubesh Karst Region (I-G) is the third area of
Western Balkan Mountain where the geodynamic
processes have a direct impact on the evolution of the
karst system (Fig.
2.29
). The studies, related to the
karst tectonics of the area (Paskalev et al.
1992
) and
to the impact of the blasts from the local quarries on
the discharge of Iskrets karst springs (Shanov et al.
2001
; Shanov and Benderev
2005
) supplied the nec-
essary data for understanding the close relationship
between the local tectonics, the tectonic stress fields
and the karst features of the region.
During the twentieth century, the springs dried up
several times. After Vranchea Earthquake from 1977
(M = 7.2) at a distance of about 400 km from the
site, the discharge rate dropped down from 5.5 to
0.5 m
3
/s during 7.5 h (Paskalev et al.
1992
). Later,
springs'
level
(Fig.
2.37
).
The
level
of
the
